Among conventional types of radiating sources with an open cavity which are excited by a dipole, sources comprising open cavities of revolution are particularly favored. Such a source, having a cylindrical open cavity, is formed by a radiating dipole placed inside a cylindrical metal base of circular cross-section. Because of the excitation phenomena of certain modes and of reflection in the cavity, the radiation diagram of such a source is formed by concentric isolevel circles up to a reduction in gain of 10 dB, beyond which the circles give way to concentric ellipses. This defect of symmetry of the radiation diagram in relation to the direction of propagation limits the bandwidth of useful frequencies.
In the case of an antenna array, the establishment of an identical radiation diagram for all directions of propagation requires a circular symmetry of the radiation diagram of each elementary source of the antenna array about its axis.